An increasing body of evidence is accumulating showing that both univalent and divalent cations play important roles in the regulation of metabolic processes. Univalent cations such as Na and K ion are involved in the transport of ions, sugars, and amino-acids in a variety of different types of living cells. At least sixty important enzymes that are involved in protein synthesis, starch synthesis, glycolysis, fatty acid metabolism, and purine and pyrimidine synthesis require univalent cations for activity. In the research program now in progress certain representative univalent cation-activated enzymes are selected as model systems and attempts are made to understand how univalent cations influence activity. Studies have been conducted with pyruvate kinase in which fluorescence polarization techniques are used to determine the influence of univalent cations on the binding of an ADP derivative to the enzyme. Also, the number of binding sites for ADP have been estimated. In another phase of this research the univalent cation requirements of various isoenzymes of B-galactosidase have been studied. The possibility that bound oligomers of the enzyme may be involved in the transport of lactose has been investigated by use of mutants of E. coli lacking components of the lactose operon.